Aggregates

Is ASR Expansion Linearly Related to Amount of Expansive Material?

Question: We want to buy a jig to float out some of the opaline shale in our sand source. The shale is reactive and we need to reduce the expansion when the sand is tested by either ASTM C 1260, "Standard Test Method for Potential Alkali Reactivity of Aggregate," or C 1293, "Standard Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction." Is there any data to show how much you would reduce expansion if you reduced the percentage of reactive material in the sand from say 3% to 1.5%?

Answer: Figure 1 shows how the amount of reactive material in the aggregate affects expansion caused by ASR. The expansion percentage rises rapidly to a peak at about 5% reactive aggregate and appears to be pretty linear. Based on this nearly linear relationship, you could assume that expansion percentage is directly related to percentage of reactive aggregate and that halving the amount of reactive material would halve the expansion percentage. However, the data for this graph weren't obtained using either ASTM C1260 or C 1293 test methods.Bryant Mather of the U.S. Army Corps of Engineers Waterways Experiment Station suggests you make test specimens for either of the tests using a 50-50 mixture of reactive and nonreactive sand. Because even the nonreactive-sand specimens will show some expansion, you'll also need specimens made with that sand only to establish a base-level expansion for it. Test both sets of specimens and compare the results with those already obtained for the reactive sand.For instance, if the reactive-sand specimen expands 0.22% and the nonreactive-sand specimen expands 0.08%, you should then expect the 50-50-blend specimen to expand 0.15%.This approach has one possible defect. There is also an aggregate-particle size effect on ASR, as shown in Figure 2. If the jig removes half of only one opaline-shale particle size, this won't be as effective as removing half of all particle sizes. Figure 2 shows that the most reactive sizes fall between the No. 30 and No. 200 sieve sizes, so removal of those sizes will be most effective.